The objective of the project is to investigate the changes in locomotor function that occur after lesion placement of different portions of the vestibular and related neural system in squirrel monkeys. Post-operative locomotor dysequilibrium and compensation, and oculomotor dysfunction and its recovery are measured by a well- controlled and repeetable procedure. Locomotor equilibrium function tests to be used are the squirrel monkey rail test and platform runway test which are based upon the behavioral conditioning procedures. These tests permit quantitative and thus comparable observations between different lesion placement situations. A comparative study on different otolithic end organ lesions (measured by the squirrel monkey platform runway test) indicated the importance of utricular input comparing to that from macula sacculi. A study on total otolithic end organ ablation showed a considerable inhibition of physically advanced locomotor performance measured by the squirrel monkey rail test. Lesion placements on vestibular nuclei area (unilateral) resulted in a severe ataxia and showed very poor locomotor functional recovery. The squirrel monkey stabilitimeter to measure the static equilibrium control is to be developed. The objective is also to investigate the effects caused by different lesion placements within the body equilibrium system, and the difference in recovery from such dysfunction. Bibliographic references: Igarashi, M.: Vestibular experiments in the squirrel monkey. Canad. J. Otolaryng. 3:334-342, 1974; Filippone, M.V., Igarashi, M., Miyata, H., Coats, A.C., and Alford, B.R.: Experimental superior vestibular nerve sectioning in squirrel monkeys. Arch. Otolaryng. 101:241-245, 1975.